In many applications, especially in the aircraft avionics environment, one of the primary power sources available is dc. This primary power is converted to secondary power within the power supply of the avionics systems. Dc-dc converter (also referred as a switching power supply) is a switching device that converts dc voltage to a square wave ac voltage. The square wave ac voltage may then be transformed to the desired voltage level and rectified to obtain dc in the power supply, or the square wave voltage may be used to directly operate ac devices. The square wave is preferred in converters, over the universally used sine wave in power distribution equipment, because of the much greater efficiency obtainable from switching devices operating in a square wave mode. Further, many ac devices intended for operation from sine waves, including rotating machinery, will operate satisfactorily with a square wave input.
Dc-dc converters are also widely used in applications where ac power is available, but where the magnetic components required in circuits operating at these ac frequencies would be prohibitively large and heavy. Again, the aircraft environment is especially suited for converter applications. Converters operating in the kilohertz range provide a substantial reduction in the size and weight of power transformers, filtering is also greatly simplified at these higher frequencies. Usually, only a small filter capacitor is required. The dc required to operate such converters is usually obtained by rectifying the available ac line voltage with a bridge rectifier, or directly from the dc primary power source thus also eliminating the need for low frequency power transformers.
Typically, dc-dc converters use semiconductor switches, either bipolar transistors or field-effect transisters to apply a dc voltage with rapidly reversing polarity to the primary winding of a transformer. A simplified converter circuit 10 is illustrated in FIG. 1. The converter circuit 10 includes two switching transistors 12 and 14 that are used in a push-pull arrangement to alternately apply a dc voltage (from a dc voltage source 11) to opposite halves of a center-tapped output transformer 16. The bases of both the transistors 12 and 14 are driven by a separate switching source, illustrated as a multivibrator timing oscillator 18, through a transformer 20. Power is delivered to the loads through the secondary winding of the transformer 16.
One well known problem associated with inverters is the leakage of high frequency currents from the output square wave signal back into the dc source. These high frequency noise signals can radiate high-frequency RF power by means of the cable supplying the power, which is then propagated to other converters or devices connected to the dc source. The problem is compounded by the constant efforts to increase the operating frequency of converters and thereby keep down the weight of the transformers (magnetics) associated with each one. Use of high-frequency HEXFET switching power supplies creates large high-frequency common-made noise signals that prior art filter designs cannot reject. For example, currently available switching power supplies or converters are operating above 60 kHz. At these high frequencies, conventional inductors are not suitable for filtering the high frequency components produced by the converters. At these high frequencies the inductor must be made much larger, creating another weight problem, and the inductor core must be made of a material other than the conventional iron to achieve suitable performance characteristics.